Test results were expected Thursday on letters possibly contaminated with the deadly poison ricin that were sent to President Barack Obama and Sen. Roger Wicker this week. The FBI arrested Paul Kevin Curtis in connection with the case at his home in Corinth, Mississippi, on Wednesday, the department said in a statement.
The letters, which were discovered on Tuesday, were addressed to Wicker, a Mississippi Republican, and to Obama. The justice department release said a third letter was sent to a Mississippi justice official.
The letters to Wicker and Obama were stopped at a government mail-screening facility after initial tests indicated the presence of ricin.
Because initial tests can be "inconsistent," the envelopes were sent off for additional tests, an FBI statement said.

The letters read: "To see a wrong and not expose it, is to become a silent partner to its continuance."
They were signed "I am KC and I approve this message," a source said.Letters put spotlight Texas chiropractor's words
The line in the letters about exposing "a wrong" comes from John Raymond Baker, a longtime Texas chiropractor, his wife said. It's been widely quoted online, but Tammy Baker sounded surprised that it was used in the letters under scrutiny in Washington.
When contacted by CNN, she said she was not aware of the letters and that the phrase refers to her husband's general philosophy of care.
She said their office phone started ringing frequently Wednesday afternoon, and it was "kind of freaking out our other employee."
A 2006 post on a blog for Baker's office said the comment originally was a criticism of insurance companies. Since then, the site said, it "has been a quote that has been picked up and quoted (sometimes without attribution) around the net" and "people are using it about all kinds of injustices."
After the arrest was announced Wednesday night, Wicker released a statement thanking "the men and women of the FBI and U.S. Capitol Police for their professionalism and decisive action in keeping our family and staff safe from harm." His offices in Mississippi and Washington "remain open for business to all Mississippians," Wicker said in the statement.
Mail for members of Congress and the White House has been handled at off-site postal facilities since the 2001 anthrax attacks, which targeted Sen. Patrick Leahy, D-Vermont, and then-Majority Leader Tom Daschle, D-South Dakota.On heightened alert
Reports of suspicious packages and envelopes also came into two Senate office buildings late Wednesday morning. Capitol Police evacuated the first floor of the Hart Senate Office Building for more than an hour and questioned a man in the area who had a backpack containing sealed envelopes, but the man was not taken into custody.
"It just reminds you that with public service comes the real possibility that you could be a target," said Sen. Pat Roberts, R-Kansas. "But on the other side of it, we have an excellent police force, and I think they'll get to the bottom of it."
White House spokesman Jay Carney said the president had been briefed on the letters.
"Obviously, he understands and we all understand that there are procedures in place, as the FBI has said. There are, there is a process in place that ensures that materials that are suspicious or substances that are found to be suspicious at remote locations are then sent for secondary and more intense testing, and that process is under way now," Carney said.
What is ricin?Beyond Washington, suspicious letters spotted
Sadie Holland, a judge in Lee County, Mississippi, told CNN on Wednesday night that she received an envelope with a suspicious substance and a letter similar to the ones sent to the offices of Obama and Wicker.
Holland said the letter included the line "Now someone must die."
Last Wednesday, the judge received and opened the typewritten letter -- postmarked from Memphis, without a return address -- that included "suspicious content," Lee County Sheriff Jim Johnson told reporters. The letter had "great consistencies and similarities" to the letters sent to Obama and Wicker, he said. Investigators were testing the contents of the letter to determine whether ricin was inside, he said.
Holland told CNN the letter originally tested negative for ricin but was being retested Wednesday, with results also expected on Thursday. Local authorities were awaiting the test results to determine whether to file state charges, Johnson said.
"The letter was handled, the chemical was handled by several different individuals in our justice court system," Johnson said, but added that "we do not have any reason to believe that anyone's life is in danger."Suspicious letters in Michigan and Arizona too
Investigators are trying to determine whether suspicious letters found at Senate offices elsewhere in the country came from the same source, federal law enforcement sources said.
Sen. Carl Levin, D-Michigan, said one of his home-state offices received a "suspicious-looking" letter and alerted authorities. "We do not know yet if the mail presented a threat," said Levin, the chairman of the Senate Armed Services Committee.
A staffer for Arizona Sen. Jeff Flake flagged "suspicious letters" at the freshman Republican's Phoenix office, Flake spokeswoman Genevieve Rozansky said in a statement, but "no dangerous material was detected in the letters."
Phoenix Fire Department spokesman Jonathan Jacobs said the envelope contained some type of powder. The person who initially found the envelope is being treated at a Phoenix-area hospital for a pre-existing condition and stress from the event, and others in the immediate vicinity were examined as well.
In a statement issued Wednesday, the FBI said it has no indication of a connection between the tainted letters and Monday's bombings at the finish line of the Boston Marathon. But the discoveries further heightened security concerns at a time when Congress is considering politically volatile legislation to toughen gun laws and reform the immigration system.Ricin is easily made
Ricin is a highly toxic substance derived from castor beans. As little as 500 micrograms, an amount the size of the head of a pin, can kill an adult. There is no specific test for exposure and no antidote once exposed.
It can be produced easily and cheaply, and authorities in several countries have investigated links between suspect extremists and ricin. But experts say it is more effective on individuals than as a weapon of mass destruction.
Ricin was used in the 1978 assassination of Bulgarian dissident Georgi Markov. The author, who had defected nine years earlier, was jabbed by the tip of an umbrella while waiting for a bus in London and died four days later.
A previous ricin scare hit the Capitol in 2004, when tests identified it in a letter in a Senate mail room that served then-Majority Leader Bill Frist's office. The discovery forced 16 employees to go through decontamination procedures, but no one reported any ill effects afterward, Frist said.

Federal authorities have produced scant evidence linking a Mississippi man to the mailing of ricin-laced letters to the president and a senator, his attorney says.
Christi McCoy said after a court hearing Friday that the government has offered no evidence to prove her client, Paul Kevin Curtis, had possession of any ricin or the seed from which it is extracted -- castor beans. An FBI agent testified during the hearing that he could not say if investigators had found ricin at Curtis' home, and McCoy said the evidence linking the 45-year-old to the crime so far has hinged on his writings posted online.
He is adamant that he did not do this, and she said she has seen nothing to prove him wrong.
Curtis was ushered into the courtroom before the hearing began in an orange jail jumpsuit and shackles. He turned to face his daughter in the audience before the hearing and whispered, "I didn't do it."
Prosecutors had wanted to delay the hearing because searches of Curtis home and car had not been completed and DNA and other tests are pending.
FBI spokeswoman Deborah Madden said Saturday that she couldn't discuss what was found during the search of Curtis' home because it's part of an ongoing investigation.
Curtis' brother Jack Curtis and 20-year-old daughter Madison Curtis watched the court proceeding Friday and said afterward they are not convinced he did what he is accused of, even though they tried to keep an open mind about what would be presented.
"After hearing what I heard in this courtroom, it appears to me that the reason I haven't been provided any evidence is there appears to be none that would link my brother directly to the charges that have been made," Jack Curtis said after the hearing.
So far, Paul Kevin Curtis is the primary focus for investigators and the only person arrested in connection with sending those letters and a third threatening letter mailed to a judge. But during a hearing Friday, FBI agent Brandon M. Grant testified that authorities were still trying to determine whether there were any co-conspirators.
As the hearing went on for roughly two hours, Grant said under questioning by Curtis' attorney that he could not say whether any ricin had been found at Curtis' home because the investigation was ongoing. Investigators had found a package they were interested in, but Grant said he did not know what was in it.
Grant testified that there were indentations on the letters from where someone had written on another envelope that had been on top of them in a stack. The indentations were analyzed under a light source and turned out to be for Curtis' former addresses in Booneville and Tupelo, Grant said.
Grant also testified that there was one fingerprint on the letter sent to the judge but that it didn't match Curtis. He said several people handled the letter, and DNA and other tests are pending.
Curtis' lawyer peppered the agent with questions in an attempt to show the government had little hard evidence, but Grant said people's lives were at risk and it wasn't like a fraud investigation in which authorities could gather more evidence before making an arrest.
Family and acquaintances have described Curtis as a caring father and enthusiastic musician who struggled for years with mental illness and who was consumed by trying to publicize his claims of a conspiracy to sell body parts on the black market.
Curtis is an Elvis impersonator and performed at parties. Friends and relatives also say he spiraled into emotional turmoil trying to get attention for his claims of uncovering a conspiracy to sell body parts on the black market.
Grant testified that Curtis' family had become increasingly concerned by his behavior.
Grant said Curtis' ex-wife told authorities that he fought with his daughter around Christmas and told her, "Maybe I should go ahead and kill you."
Madison Curtis said after the hearing that she loves her father and stands by him.
Grant also testified that Curtis' ex-wife said Curtis once told her that he was in hostage situation in Chicago after a breaking up with a former girlfriend, threatened suicide and shot a gun in the air.
However, the agent said they haven't been able to find a record of that.
Grant's testimony ended Friday evening, but the hearing is set to continue Monday morning.
In court documents, Curtis' attorney, Christi McCoy, gave some details of Curtis' arrest. Curtis had gone to get his mail outside his home and was planning to go to his ex-wife's home to cook dinner for her and their children when he was approached by officers in SWAT gear, she wrote. He was then interrogated at an FBI office for several hours, handcuffed and chained to a chair.
Curtis cooperated to the best of his ability, but when he suggested he might need a lawyer, an agent discouraged that, McCoy wrote.
According to an FBI affidavit, the letters he sent read: "Maybe I have your attention now even if that means someone must die."
Officials have confirmed that the letters contained ricin.
While the toxin can be extremely lethal in its purest form, experts say more crude forms are relatively easy to make.
The FBI has not yet revealed details about how the ricin was made or how lethal it may have been. It was in a powdered form inside the envelopes, but the FBI said no one has been sickened by it so far. A senate official said Thursday that the ricin was not weaponized, meaning it wasn't in a form that could easily enter the body.
More than a dozen officials, some wearing hazardous materials suits, were searching the home Friday where Curtis was arrested in Corinth, Miss. FBI spokeswoman Deborah Madden would not say if authorities have found ricin or materials used to make it in Curtis' home, and officials have not provided details about how Curtis may have either obtained or made the ricin.
Curtis' ex-wife has said he likely didn't have the know-how to make ricin, and she did not know where he would buy it because he was on disability.
But ricin was once known as "the poor man's bioterrorism" because the seeds are easy to obtain and the extraction process is relatively simple, said Murray Cohen, the founder of the Atlanta-based Frontline Foundation, which trains workers on preparedness and response to bioterrorism and epidemics.
"Any kid that made it through high school science lab is more than equipped to successfully make a poison out of this stuff. Any fool can get recipes off the Internet and figure out how to do it," Cohen said.
Those seeds, which look a bit like coffee beans, are easy to buy online and are grown around the world; they are often used to make medicinal castor oil, among other things. However, using the seeds to make a highly concentrated form of ricin would require laboratory equipment and expertise to extract, said Raymond Zilinskas, a chemical and biological weapons expert.

Investigators haven't found any ricin in the house of Mississippi man accused of mailing poisoned letters to President Barack Obama, a U.S. senator and a local judge, according to testimony Monday from an FBI agent.
Agent Brandon Grant said that a search of Paul Kevin Curtis' vehicle and house in Corinth, Miss., on Friday did not turn up ricin or ingredients for the poison. A search of Curtis' computers has found no evidence so far that he researched making ricin.
"There was no apparent ricin, castor beans or any material there that could be used for the manufacturing, like a blender or something," Grant testified. He speculated that Curtis could have thrown away the processor. Grant said computer technicians are now doing a "deep dive" on the suspect's computers after initially finding no "dirty words" indicating Curtis had searched for information on ricin.

Charges were dropped Tuesday against the Mississippi man accused of sending ricin-laced letters to President Barack Obama and others, while authorities searched at another man's home in connection with the case.
The surprising move was announced in a brief document filed in federal court in Oxford hours after Paul Kevin Curtis was released from custody. The charges were dismissed without prejudice, which means they could be re-instated if prosecutors so choose.
Attorneys for Curtis have suggested he was framed, and an FBI agent testified in court this week that no evidence of ricin was found in searches of his home. At a news conference Tuesday, they declined to discuss whether they were told what new information the government had uncovered.
"I respect President Obama," Curtis said to reporters. "I love my country and would never do anything to pose a threat to him or any other U.S. official."
Prosecutors couldn't immediately be reached for comment.
In Tupelo, numerous law enforcement officers converged on the home of another Mississippi man, including some in hazmat suits.
Everett Dutsche said in a phone interview with The Associated Press that the FBI was at his Tupelo home Tuesday for the search connected to the mailing of poisoned letters to Obama, U.S. Senator Roger Wicker of Mississippi and a state judge. Dutschke said his house was also searched last week.
Dutschke has maintained his innocence and says he doesn't know anything about the ingredients for ricin. He said agents asked him about Curtis, whether Dutschke would take a lie detector test and if he had ever bought castor beans, which can be used to make the potent poison.
"I'm a patriotic American. I don't have any grudges against anybody. I did not send the letters," said Dutschke, who hasn't been arrested or charged.
After charges were dropped against Curtis, he said: "I'm a little shocked."
Tuesday's events began when the third day of a preliminary and detention hearing was cancelled without officials explaining the change. Within two hours, Curtis had been released, though it wasn't clear why at first.
Through his lawyers, Curtis has denied involvement in the letters.
"The searches are concluded, not one single shred of evidence was found to indicate Kevin could have done this," Defense lawyer Christi McCoy told reporters after a hearing Monday.
McCoy said in court that someone may have framed Curtis. She questioned why Curtis would have signed the letters "I am KC and I approve this message," a phrase he had used on his Facebook page.
Later, at the news conference, Curtis said the past week had been a nightmare for his family.
Referring to questioning by investigators, Curtis said: "I thought they said rice, and I said `I don't even eat rice."'
FBI Agent Brandon Grant said in court on Monday that searches last week of Curtis' vehicle and house in Corinth, Miss., found no ricin, ingredients for the poison, or devices used to make it. A search of Curtis' computers found no evidence he researched making ricin. Authorities produced no other physical evidence at the hearings tying Curtis to the letters.
Curtis was arrested last Wednesday at his house in Corinth, Miss. The first of the letters was found two days earlier.
Grant testified Friday that authorities tried to track down the sender of the letters by using a list of Wicker's constituents with the initials KC, the same initials in the letters. Grant said the list was whittled from thousands to about 100 when investigators isolated the ones who lived in an area that would have a Memphis, Tenn., postmark, which includes many places in north Mississippi. He said Wicker's staff recognized Curtis as someone who had written the senator before.
All the envelopes and stamps were self-adhesive, Grant said Monday, meaning they won't yield DNA evidence. He said thus far the envelopes and letters haven't yielded any fingerprints.
On Tuesday, people in hazmat suits were seen going in and out of Dutschke's house on a quiet block in Tupelo. Investigators from the FBI, U.S. Marshals Service and U.S. Capitol Police were seen outside the house. Dutschke said he counted at least 30 law enforcement agents.
Dutschke said his attorney wasn't with him and he didn't know whether he was going to be arrested
Dutschke said that he knows Curtis but that the two had a falling out. Dutschke said the last contact they had was in 2010 when Dutschke threatened to sue Curtis for saying he was a member of Mensa, a membership group for people with high IQs.